This application claims the priority of Japanese Patent Application No. 11-080822 filed on Mar. 25, 1999, which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an X-ray analyzer, mounted to a scanning electron microscope (SEM) or the like, for carrying out element analysis of a specimen according to characteristic X-rays emitted therefrom upon irradiation with electron beams; and an analyzing method using the same.
2. Description of the Prior Art
Among specimen observing apparatus such as scanning electron microscope (SEM) and the like, there have been known those which qualitatively or quantitatively analyze elements contained in a specimen by analyzing a spectrum of characteristic X-rays generated from the specimen upon irradiation with electron beams.
When elements contained in the specimen are to be analyzed quantitatively, it has been desired that, since the irradiation current value of electron beams with respect to the specimen fluctuates greatly, this value be monitored and the measured X-ray dose be calibrated based thereon in an energy dispersive X-ray analyzer, for example, so as to improve the accuracy in quantitative analysis.
However, it is not always easy to correctly measure the irradiation current value during the final analysis of the specimen. Therefore, in conventional techniques, it has been assumed, for example, that the irradiation current value of electron beams in SEM does not vary in a short period such as an analyzing time, and the irradiation current value is measured by use of a Faraday cage, or the irradiation current value is computed according to the measured X-ray dose of a specific reference sample, so as to determine the irradiation current value during the final analysis of the specimen.
However, the irradiation current value of electron beams in an SEM varies in a short period of time such as the final measuring time in practice, such that the assumed irradiation current value of electron beams differs from the actual one, thereby making it difficult to greatly improve the accuracy in quantitative analysis.
In view of such circumstances, it is an object of the present invention to provide an X-ray analyzer with a simple configuration which can determine an accurate irradiation current value of electron beams during measurement of a specimen when carrying out quantitative analysis of elements contained in the specimen by detecting characteristic X-rays generated from the specimen upon irradiation with the electron beams, thereby greatly improving the accuracy in analysis of elements contained in the specimen; and an analyzing method using the same.
In SEMs equipped with a field emission type (FEG type) electron gun, since the irradiation current of electron beams fluctuates more greatly than in other SEMs, it is necessary to monitor the irradiation current of electron beams during the final measurement of the specimen even if the configuration of the apparatus is somewhat complicated thereby.
Therefore, conventional FEG type SEMs use irradiation current measuring means or the like comprising a Faraday cage provided with an aperture for transmitting electron beams or the like, disposed on the electron beam source side from the specimen, so as to monitor a part of irradiation current of electron beams during the final measurement of the specimen, and calibrate the measured X-ray dose based thereon. In the FEG type SEMs, however, the cross-sectional intensity distribution of the irradiation current flux changes with time because of the fact that the surface state of the filament of the electron gun changes with time, and so forth, so that the irradiation current value detected by the irradiation current measuring means or the like is not proportional to the actual irradiation current value upon the specimen, whereby it has been difficult to greatly improve the accuracy in quantitative analysis.
In view of such circumstances, it is an object of the present invention to provide an X-ray analyzer which can determine a correct irradiation current value of electron beams during measurement of a specimen even when carrying out quantitative analysis of elements contained in the specimen by detecting characteristic X-rays generated from the specimen upon irradiation with the electron beams in an FEG type SEM, thereby greatly improving its accuracy in analysis; and an analyzing method using the same.
The present invention achieves the above-mentioned objects by monitoring an absorption current value which has a good proportional relationship with the irradiation current value of an electron beam irradiating the specimen, and determining the irradiation current value of the electron beam according to the result of monitoring.
Namely, a first X-ray analyzer in accordance with the present invention is an X-ray analyzer comprising an electron beam source for generating an electron beam, electron beam converging means for converging the electron beam onto a specimen, and an X-ray detecting section for detecting a characteristic X-ray generated from the specimen upon irradiation with the electron beam;
the X-ray analyzer comprising:
an absorption current measuring section for measuring an absorption current value from the specimen and measuring an absorption current value from a reference sample disposed so as to be interchangeable with the specimen;
a proportionality factor calculating section for calculating a value of ratio A between respective absorption current values in the specimen and reference sample; and
reference sample absorption current calculating means for calculating, when the specimen is finally measured, an absorption current value of the reference sample at each measuring time by multiplying each measured absorption current value of the specimen by the value of ratio A determined by the proportionality factor calculating section.
A second X-ray analyzer in accordance with the present invention is an X-ray analyzer comprising an electron beam source for generating an electron beam, electron beam converging means for converging the electron beam onto a specimen, and an X-ray detecting section for detecting a characteristic X-ray generated from the specimen upon irradiation with the electron beam;
the X-ray analyzer comprising:
an absorption current measuring section for measuring an absorption current value from the specimen;
an irradiation current measuring section disposed at a part of positions where the electron beam passes;
a proportionality factor calculating section for calculating a value of ratio C between the absorption current value in the specimen measured by the absorption current measuring section and an irradiation current value measured by the irradiation current measuring section substantially simultaneously with when the absorption current value is measured by the absorption current measuring section; and
means for calculating, when the specimen is finally measured, an irradiation current value to be measured by the irradiation current measuring section at each measuring time from an absorption current value in the specimen or a reference sample disposed interchangeable with the specimen by multiplying each measured absorption current value of the specimen by the value of ratio C determined by the proportionality factor calculating section.
A third X-ray analyzer in accordance with the present invention is an X-ray analyzer comprising an electron beam source for generating an electron beam, electron beam converging means for converging the electron beam onto a specimen, and an X-ray detecting section for detecting a characteristic X-ray generated from the specimen upon irradiation with the electron beam;
the X-ray analyzer comprising:
an absorption current measuring section for measuring an absorption current value from the specimen and measuring an absorption current value from a reference sample disposed so as to be interchangeable with the specimen;
an irradiation current measuring section disposed at a part of positions where the electron beam passes;
a current value ratio calculating section for calculating a value of ratio C between the absorption current value measured by the absorption current measuring section and an irradiation current value measured by the irradiation current measuring section substantially simultaneously with when the absorption current value is measured by the absorption current measuring section; and
an apparatus comprising a storage section for storing a plurality of values of C,
the apparatus comprising:
a current value ratio calculating and storing section for determining and storing respective ratio values Cstd, Csmp for the reference sample and specimen,
a proportionality factor calculating section for determining a value of ratio K between the stored Cstd and Csmp, and
reference sample absorption current calculating means for calculating, when the specimen is finally measured, an absorption current value of the reference sample at each measuring time by multiplying each measured absorption current value of the specimen by the value of ratio K determined by the proportionality factor calculating section.
A first analyzing method in accordance with the present invention is a method comprising the steps of irradiating a specimen with an electron beam generated from an electron beam source and converged onto the specimen, detecting a characteristic X-ray generated from the specimen upon irradiation with the electron beam, and analyzing an element contained in the specimen according to a result of the detection;
the method comprising the steps of:
before finally measuring the specimen, successively measuring an absorption current value from the specimen according to the electron beam irradiation and an absorption current value obtained from a reference sample disposed interchangeably with the specimen according to irradiation of the reference sample with the electron beam;
determining a value of ratio A between the absorption current value from the specimen and the absorption current value from the reference sample; and then
when finally measuring the specimen, while measuring an absorption current value from the specimen according to the electron beam irradiation, multiplying thus measured absorption current value by the proportionality factor A, and using a result of the multiplication as a substitute for the irradiation current value for the specimen, thereby analyzing the element contained in the specimen.
A second analyzing method in accordance with the present invention is a method comprising the steps of irradiating a specimen with an electron beam generated from an electron beam source and converged onto the specimen, detecting a characteristic X-ray generated from the specimen upon irradiation with the electron beam, and analyzing an element contained in the specimen according to a en result of the detection;
the method comprising the steps of:
before finally measuring the specimen, measuring an absorption current value from the specimen according to the electron beam irradiation and, substantially simultaneously therewith, measuring an irradiation current value caused by the electron beam at a position where the electron beam passes;
determining a value of ratio C between thus measured values; and then
when finally measuring the specimen, while measuring an absorption current value from the specimen according to the electron beam irradiation, multiplying thus measured absorption current value by the value of ratio C, and using a result of the multiplication as a substitute for the irradiation current value for the specimen, thereby analyzing the element contained in the specimen.
A third analyzing method in accordance with the present invention is a method comprising the steps of irradiating a specimen with an electron beam generated from an electron beam source and converged onto the specimen, detecting a characteristic X-ray generated from the specimen upon irradiation with the electron beam, and analyzing an element contained in the specimen according to a result of the detection;
the method comprising the steps of:
before finally measuring the specimen, measuring an absorption current value from a reference sample according to the electron beam irradiation and, substantially simultaneously therewith, measuring an irradiation current value caused by the electron beam at a position where the electron beam passes;
determining and storing a value of ratio Cstd between thus measured values;
measuring an absorption current value from the specimen according to the electron beam irradiation and, substantially simultaneously therewith, measuring an irradiation current value caused by the electron beam at a position where the electron beam passes;
determining and storing a value of ratio Csmp between thus measured values;
determining a value of ratio K, as a proportionality factor, between the value of ratio Cstd for the reference sample and the value of ratio Csmp for the specimen; and then
when finally measuring the specimen, while measuring an absorption current value from the specimen according to the electron beam irradiation, multiplying thus measured absorption current value by the proportionality factor K, and using a result of the multiplication as a substitute for the irradiation current value for the specimen, thereby analyzing the element contained in the specimen.
In this specification, xe2x80x9cabsorption currentxe2x80x9d refers to, of the irradiation current of the electron beam irradiating the specimen and the like, the current absorbed in the specimen and the like without being released into the space.
In the X-ray analyzer and the analyzing method using the same in accordance with the present invention, as mentioned above, the absorption current value having a predetermined proportional relationship with the irradiation current value of the electron beam irradiating the specimen is measured before the final measurement, and the irradiation current value of the electron beam is monitored at the final measurement according to the result of measurement.
Thus constituted present invention is based on the following ideas:
Since the above-mentioned absorption current greatly varies due to the kind of specimen measured and the surface state thereof, it is hard to monitor a correct irradiation current value even when the absorption current Value is used as it is.
For example, in the case where specimens different from each other are irradiated with the same intensity of irradiation current, the respective absorption current values from these specimens may differ from each other in general.
As a consequence, letting Ia be the intensity of current value absorbed by a specimen upon irradiation with an irradiation current with an intensity Io, their proportionality factor Ac (=Io/Ia) varies for each specimen in general.
However, as long as the same specimen is measured, the absorption current value has a good proportional relationship with the irradiation current value when its surface state does not vary. Therefore, when the proportionality factor Ac for each specimen is calculated before the final measurement, and the measured absorption current value of the specimen at the final measurement is multiplied by the proportionality factor Ac, the irradiation current value that has conventionally been unavailable unless being measured with a Faraday cage or the like can be monitored over the whole period of the final measurement.
Namely, an absorption current value (Istd) in a reference sample is measured and, immediately thereafter, a specimen is set at a measuring position in place of the reference sample, so as to measure an absorption current value (Ismp). Thereafter, both of thus measured current values are used for determining a proportionality factor A (=Istd/Ismp). Subsequently, the absorption current value of the specimen is measured during the final measurement, and thus measured value is multiplied by A, whereby data identical to that measured when the absorption current of the reference sample is measured can be obtained.
Assuming that the relationship between the absorption current value of the reference sample and the irradiation current value of an electron beam therefor has been known beforehand, if the above-mentioned proportionality factor is determined before the final measurement of each specimen, the irradiation current value of the electron beam at the time of measurement can be calculated during the final measurement for any specimen once its absorption current value is measured.
Feeding back thus obtained irradiation current value data to the X-ray analyzer can improve the accuracy of quantitative analysis.
Instead of the above-mentioned proportionality factor A, the absorption current value of the specimen and the irradiation current value at that time may be measured before the final measurement, and the proportionality factor C may be determined by use of thus measured values, whereby effects similar to those mentioned above can be obtained.